Technical Field

This invention relates to a water-based method for cleaning metal surfac¬ es such as, most importantly, the surfaces of iron, zinc, aluminum, and the like. More particularly, with the goal of implementing environmental countermeasures such as protection of the ozone layer, this invention relates to a method for cleaning metal surfaces with water-based compositions that can replace the use of solvents, such as Freon™, trichloroethane, and the like. Statement of Related Art

Organic solvents, mainly Freon, trichloroethane, and the like, have hereto- fore been used in the metal surface cleaning operations that are frequently imple¬ mented as a pre-treatment to metal working, painting, heat treatment, conversion treatment, and the like. This practice is followed because it offers a number of advantages, such as: (1 ) strong ability to dissolve oily contaminants, (2) easy drying,

(3) no risk of corrosion since there is no contact with water,

(4) simple cleaning equipment that does not require a large amount of space, and

(5) no requirement for waste water treatment facilities. However, because the release of these solvents into the atmosphere is related to destruction of the earth's ozone layer, in the last few years there has been a widening global movement to control their production and use. This has led to the current strong and urgent demand for water-based cleaning proced¬ ures that do not employ the aforesaid solvents. Water-based cleaning is typically carried out using at least the following four steps: (1 ) cleaning, (2) a water rinse, (3) a corrosion-preventive rinse, and (4) drying. The cleaning step uses a clean¬ ing composition (often called hereinafter a "bath" for brevity, even if used by spraying or some other method than immersion) that contains components such as surfactant and inorganic builder, while the subsequent water rinse functions

to wash off residual cleaning bath present on the material surface. In the ensu¬ ing corrosion-preventive rinse step the workpiece is treated with a rinse agent, which contains components such as corrosion-preventive agent and water-drain¬ ing aid, since the workpiece would otherwise corrode or discolor if dried directly after the water wash. The final drying step involves drying with, for example, hot air.

Since the cleaning bath enters and thereby contaminates the rinse water and corrosion-preventive rinse bath during the execution of this typical proced¬ ure, the rinse water and corrosion-preventive rinse bath must be intermittently or continuously discharged during treatment. This effluent is treated in a separately installed effluent treatment facility.

A non-rinse method is available which omits the water rinse step and cor¬ rosion-preventive rinse step in pursuit of the goals of reducing the space required by the facility and avoiding effluent discharge from cleaning. This method can be used, however, only when a low cleaning performance is required, because it leaves the surface of the workpiece contaminated with the oil and other com¬ ponents (such as surfactant and alkali) that are present in the cleaning bath.

Moreover, the heretofore practiced system as described above requires a relatively large facility space since it requires a large number of treatment baths. As a result, even when the decision has been made to change a conven¬ tional 1- or 2-bath solvent-based cleaning facility over to a water-based cleaning operation, introduction of the equipment must often be abandoned due to inade¬ quate facility space. An additional problem is that the necessary effluent treat¬ ment facility frequently is not already present. The large cost burden associated with the installation of a new effluent treatment facility will often cause conversion to a water-based system to be ruled out. Problems to Be Solved bv the Invention

The present invention takes as its object the introduction of a water-based method for cleaning metal surfaces which does not produce effluent from clean- ing and which employs a small-scale 1- or 2-bath water-based cleaning facility to yield a thorough level of surface cleaning that has heretofore been quite diffi¬ cult to achieve even by large-scale facilities with 3 or more baths.

Summary of the Invention

The inventors, after taking note of the fact that the cleaning effluent is pro¬ duced in the water rinse step and corrosion-preventive rinse step that follow the water-based cleaning step, discovered a novel, cleaning-effluent-free process in which the rinse water is constantly regenerated and the water rinse and corro¬ sion-preventive rinse are run simultaneously in a single process without the use of a rinse agent. This was achieved through the use of an ultrafiltration mem¬ brane as a selectively permeable membrane and through the use of a water-sol¬ uble corrosion preventive that can cross the ultrafiltration membrane. In specific terms, the present invention introduces a method for cleaning metal surfaces that characteristically comprises, preferably consists essentially of, or more preferably consists of, steps of: (a) cleaning the metal surface with an aqueous cleaning bath that contains an ultrafilter-permeable water-soluble corrosion-preventive agent, (b) subjecting the cleaning bath to ultrafiltration after cleaning to produce a filtrate, and

(c) rinsing the cleaned metal surface with the filtrate afforded by the ultrafiltra¬ tion of the cleaning bath.

The present invention also introduces a method for cleaning metal surfac- es that characteristically comprises, preferably consists essentially of, or more preferably consists of, steps of:

(a) cleaning the metal surface with a water-borne cleaning bath that contains an ultrafilter-permeable water-soluble corrosion-preventive agent,

(b) rinsing with water, (c) subjecting the rinse water to ultrafiltration after the water rinse to produce a filtrate, and

(d) rinsing the cleaned metal surface with the filtrate afforded by the ultrafiltra¬ tion of the rinse water.

Brief Description of the Drawing Figure 1 contains flow charts of some processes according to the present invention. Details of the Invention and Its Preferred Embodiments

No specific limitations apply to metals which may be subjected to the pres¬ ent invention, but the main target metals will be iron, zinc, aluminum, and the like. The type of ultrafiltration membrane used by the present invention is also not narrowly restricted. The ultrafilter-permeable water-soluble corrosion-preventive agent used by the present invention is exemplified by at least one selection from the group consisting of carboxylic acid salts, nitrites, molybdates, polyphosphate salts, bor- ate salts, sulfonate salts, phosphonate salts, amines, triazoles, thiazoles, and imidazoles. The ultrafilter-permeable water-soluble corrosion-preventive agent is spe¬ cifically exemplified by at least one selection from carboxylic acid salts such as potassium oleate, sodium stearate, sodium alkyl ether carboxylates, and the like; nitrites such as sodium nitrite and the like; molybdates; polyphosphate salts; bor- ate salts; sulfonate salts such as sodium benzenesulfonate and the like; phos- phonate salts such as sodium methylenephosphonate and the like; amines such as alkanolamine and the like; benzotriazole; mercaptobenzothiazole; imidazole; and the like.

The preferred strategy for selecting a water-soluble corrosion-preventive agent from the above-listed compounds is to select a corrosion-preventive agent matched to the particular material being cleaned. For example, the fatty acid soaps, nitrites, polyphosphate salts, amines, and molybdates are especially suit¬ able for iron and steel; the amines and phosphonate salts are especially suitable for aluminum alloys; and the triazoles and thiazoles are especially suitable for copper alloys. The cleaning agent according to the present invention will generally be an alkaline or neutral liquid cleaning bath, but it should not contain, for example, a polymeric colloid of a type and in an amount capable of substantially clogging the ultrafiltration membrane or an organic solvent of a type and in an amount capable of substantially damaging the ultrafiltration membrane. Optionally and usually preferably, it contains a water-insoluble corrosion-preventive agent along with the necessary water-soluble corrosion- preventive agent.

As long as the conditions described above are satisfied, no other narrow

limitations apply to the components (inorganic builder, surfactant, and the like) present in the water-based cleaning agent or to their proportions.

The water-based cleaning process according to the present invention con¬ sists of cleaning the metal surface with a cleaning bath and, optionally, also with rinse water, continuously ultrafiltering the post-cleaning cleaning bath and/or rinse water, and spraying the resulting ultrafiltrate on the surface of the metal after it has undergone the cleaning step, if there is no separate rinse step with water from another source than an ultrafiltrate produced as part of the method, or after such a separate rinse step if one is used. Through the addition of an ul- trafilter-permeable water-soluble corrosion-preventive agent to the components of the cleaning bath used in this process, said process yields a clean and cor¬ rosion-free surface entirely without the use of the heretofore required corro¬ sion-preventive rinse agent in a separate process step.

The components of the cleaning agent in the present process must in- elude a corrosion-preventive agent that can pass through the ultrafiltration mem¬ brane. Corrosion will readily develop on the metal in processes subsequent to rinsing with the filtrate when this corrosion-preventive agent is not present or when it is present but unable to pass through the ultrafiltration membrane.

The cleaning bath is directly subjected to ultrafiltration in the one-bath sys- tern obtained by omitting a separate rinse step. However, it is the liquid from the rinse bath that is subjected to ultrafiltration in the case of the two-bath system ob¬ tained by the utilization of a rinse bath downstream from the cleaning bath. Flow charts of the processes employed by the present invention are given in Figure 1. Figure 1(a) concerns the one-bath system that is obtained by omitting the rinse step. Figure 1(b) concerns the two-bath system, i.e., where the rinse step is present. In the case of (a), the liquid carrying concentrated contaminants as a result of ultrafiltration is returned to the cleaning bath, while in (b) the contam¬ inant-concentrated liquid can be returned to either the rinse bath or to the clean¬ ing bath. The essential characteristics of the present invention consist of the use of an ultrafiltration membrane as a process device and the use of cleaning agent that contains corrosion-preventive agent that is able to cross said ultrafiltration

membrane. Only a system that combines these two features can solve the prob¬ lems associated with the prior-art water-based cleaning methods. Thus, by itself a method that attempts closure, i.e., no effluent production, through the use of an ultrafiltration membrane will still suffer from the problem of corrosion develop- ment after the water wash or drying. On the other hand, by itself the use of a corrosion-preventive-containing cleaning bath still suffers from the problems of effluent production and a large number of processes.

The water-based cleaning method according to the present invention is able to provide a thoroughly cleaned workpiece surface because final rinsing is carried out using the clean solution yielded by the constant ultrafiltrative removal of the oil and dirt present in the cleaning bath or rinse water. In addition, since the corrosion-preventive agent, with its high corrosion-preventive rinsing activity, is selectively transferred into the filtrate, said filtrate is able to exercise a corro¬ sion-preventive rinsing activity identical to that for the use of rinse agent, but without the separate addition of rinse agent.

The content of the present invention will be explained in greater detail below with reference to several examples and comparative examples of actual cleaning methods.

Substrates The following were used: cold-rolled steel sheet (JIS SPCC-SD), alumin¬ um sheet, and copper sheet, in each case coated with 2 grams per square meter (hereinafter usually abbreviated "g/m ² ") of corrosion-preventive oil (Nox-Rust™ 530F60 from Parker Kosan Kabushiki Kaisha).

Ultrafiltration membrane NTU-2120 from Nitto Denko Kabushiki Kaisha.

Water-based cleaning bath

The cleaning bath consisted of a weakly alkaline cleaning bath (pH 9.0) containing 1.5 gram per liter (hereinafter usually abbreviated "g/L") of polyoxyeth- ylene-type nonionic surfactant, to which the water-soluble corrosion-preventive agent reported in Table 1 and 5 g/L of the aforesaid corrosion-preventive oil had also been added.

Cleaning process

Spraying with the water-based cleaning bath at 50° C for 3 minutes.

Ultrafiltration of the water-based cleaning bath.

Spray rinsing with permeate (filtrate) from ultrafiltration at ambient temperature for 30 seconds.

Hot-air drying at 120° C for 10 minutes. Evaluation methods

(1) Corrosion-preventive activity

After drying the test sheet was visually evaluated for the development of rust or discoloration.

(2) Cleaning activity The oil remaining on the surface was measured as the carbon add-on using a surface carbon analyzer (RC-212 from Leco Company).

Examples 1 to 13

Metal surfaces were cleaned by cleaning methods according to the pres¬ ent invention using different corrosion-preventive agents in the cleaning bath and different metal substrates, and the corresponding corrosion-preventive and cleaning activities were evaluated.

Comparative Example 1

In this case, the substrate metal was steel, no corrosion-preventive agent was added, and neither a water rinse nor a corrosion-preventive agent rinse were carried out.

Comparative Example 2

In this case, the substrate metal was steel, no corrosion-preventive agent was added, and rinsing was carried out with the filtrate from cleaning.

Comparative Example 3 In this case, the substrate metal was steel, a corrosion-preventive agent that would not cross the ultrafiltration membrane was added, and rinsing was

carried out with the filtrate from cleaning. Comparative Example 4

The corrosion-preventive and cleaning activities were evaluated as in Comparative Example 2 with the exception that copper was used instead of steel.

Comparative Example 5

The corrosion-preventive and cleaning activities were evaluated as in Comparative Example 3 with the exception that copper was used in place of steel. Further details of the materials used and of the results obtained in the

Examples and Comparative Examples are given in Table 1.

Table 1

No. Water-Soluble Corrosion- Rinse Substrate SCA, Corro¬ Preventive Agent with: Metal mg/m ² sion?

Ex l 1 g/L of sodium oleate* UFfFR Fe 4.5 No

Ex 2 0.2 g/L of sodium stearate* UFfFR Fe 4.2 No

Ex 3 0.5 g/L of sodium nitrite* UFfFR Fe 4.0 No

Ex 4 2 g/L of triethanolamine* UFfFR Fe 5.3 No

Ex 5 1 g/L of ammonium molybdate* UFfFR Fe 4.2 No

Ex 6 0.5 g/L of sodium pyrophosphate* UFfFR Fe 4.3 No

Ex 7 5 g/L of sodium borate* + UFfFR Fe 5.0 No 1 g/L of sodium oleate*

Ex 8 2 g/L of monoethanolamine* + UFfFR Al 6.5 No 1 g/L of triethanolamine*

Ex 9 1.5 g/L of sodium methylenephos- UFfFR Al 4.0 No phonate*

Ex 10 1 g/L of sodium benzenesulfonate* UFfFR Fe 4.5 No

Ex 11 3 g/L o f sodium alkyl ether UFfFR Fe 5.2 No carboxylate*

Ex 12 2 g/L of mercaptobenzothiazole* UFfFR Cu 6.2 No

Ex 13 2 g/L of benzotriazole* UFfFR Cu 5.3 No

CE 1 None None Fe 85 No

CE 2 None UFfFR Fe 4.1 Yes

CE 3 1 g/L of sorbitan tristearate** UFfFR Fe 4.5 Yes

CE 4 None UFfFR Cu 3.5 Disclr

CE 5 1 g/L of sorbitan tristearate** UFfFR Cu 3.7 Disclr

Footnotes for Table 1 •Permeable through the membrane used. *Impermeable through the membrane used.

Abbreviations for Table 1 No. = Number; SCA + Surface Carbon Add-on; mg/m ² = milligrams per square meter; UFfFR = Ultrafiltrate from First Rinse; Ex = Example; CE = Comparison Example; Disclr = Discoloration.

The results obtained in Examples 1 to 13 and Comparative Examples 1 to 5 support the following conclusions:

1. No corrosion developed after drying in the cleaning methods according to the present invention (Examples 1 to 13). In addition, Examples 1 to 13 gave small values for the surface carbon add-on, which was indicative of a good cleaning activity.

2. The surface carbon add-on value was high in Comparative Example 1 , in the absence of any water-soluble corrosion-preventive agent, water rinse, or corrosion-preventive rinse; this was indicative of a poor cleaning activity.

3. Corrosion or discoloration appeared in Comparative Examples 2 and 4, which had no addition of water-soluble corrosion-preventive agent, and in Comparative Examples 3 and 5, with addition to the water-based cleaning agent of sorbitan tristearate, which is a membrane-impermeable corrosion-preventive agent.

Benefits of the Invention

Metal cleaning by the water-based cleaning method according to the pres¬ ent invention can easily be used as a replacement method, even when convert¬ ing a conventional solvent-based cleaning operation with trichloroethane, Fre- on™, or the like to a water-based cleaning operation, without triggering problems such as corrosion development, effluent treatment, or lack of available equipment space. Moreover, the method according to the present invention is also an in¬ dustrially useful cleaning method because it makes possible a reduction in oper¬ ating costs, inasmuch as it requires neither rinse agent nor expenditures for ef- fluent treatment.